1. Field of the Invention
The invention relates to processes related to the production of ethylbenzene and propylene, particularly to processes related to the production of ethylbenzene and propylene from dilute sources of ethylene.
2. Description of the Related Art
Ethylene is common chemical which may be, among other things, reacted with aromatics to produce alkylaromatics, such as ethylbenzene, and with butenes to produce propylene. Ethylbenzene is commonly used to produce styrene, which may be polymerized to produce polystyrene. Propylene is commonly used for the manufacture of polypropylene.
The process for producing high purity ethylene is well known, and involves pyrolysis of a hydrocarbon feed and subsequent separation of ethylene and reaction by-products by distillation. The process generally comprises the following: A feedstock comprising ethane, propane, butane, naphtha, gas oils or hydrocracked vacuum gas oils is fed to an ethylene plant, where it is thermally cracked in the presence of steam in a bank of pyrolysis furnaces. An olefin-bearing effluent gas is formed and is quenched progressively by generating steam and through indirect contact with oil and/or water. The effluent is compressed in a multi-stage centrifugal compressor, acid gases are removed by amine treating and/or a caustic wash, and then the gases are dried over a molecular sieve. Methane offgas is recovered under cryogenic conditions in a demethanizer. Ethylene and ethane are then recovered together in a deethanizer. Acetylene is normally catalytically removed and then an ethylene product recovery takes place under low temperature conditions in a final fractionation column. Just prior to final fractionation the ethylene stream will include significant amounts of ethane (15 to 35%) and relatively small amounts of hydrogen, methane and propylene. Final fractionation results in a high purity (polymer-grade) ethylene (at least about 99.95 mol %) and recycle ethane, which may be used to produce more ethylene.
The final fractionation of the ethylene mixture is relatively energy intensive and it would be preferable to reduce the amount of ethylene/ethane processed in this manner, or to eliminate this step altogether. However, many processes, including those to produce propylene and ethylbenzene, typically are carried out with a feed of high purity ethylene. Ethylene streams diverted from the ethylene plant, after acetylene removal but before final fractionation, typically contain only about 65 mol % ethylene when ethane crackers are the source of the ethylene, and about 85 mol % ethylene when naphtha crackers are the source of the ethylene; the primary difference between the two processes being the feedstock used and a somewhat simpler recovery section for the ethane cracker (i.e. the ethane cracker has fewer distillation columns since heavy byproduct formation is reduced).
A number of processes for producing alkylaromatics, such as ethylbenzene, are also known, and may employ fixed-bed or catalytic distillation type processes. The fixed-bed process generally comprises the following: Benzene is sent to an alkylator containing a fixed bed of alkylation catalyst and reacted with ethylene to yield a mixture of alkylated benzenes and excess benzene. The mixture is fractionated to recover ethylbenzene, recycle benzene, and higher ethylated benzenes. The recycle benzene is sent back to the alkylator to react with additional ethylene and to a transalkylator, where the higher ethylated benzenes are transalkylated with the benzene to form additional ethylbenzene.
While polymer-grade ethylene is preferable for these processes, ethylbenzene can also be produced from relatively dilute ethylene feeds. In this event, catalytic distillation reactors are preferred because ethylene feeds as dilute as about 15 mol % can be utilized to produce ethylbenzene. If the fixed-bed process is used with dilute ethylene feeds, ethylene with a purity as low as about 60 mol % can be used, provided the remaining 40 mol % of the feed contains minimal hydrogen and methane content. Dilute ethylene from an ethane cracker may have relatively low amounts of methane and hydrogen, but this may not always be the case since, for example, dilute ethylene from an ethylene plant with a front-end deethanizer may contain larger quantities of hydrogen and methane. Alternatively, dilute ethylene from a fluid catalytic cracker (FCC) may contain very large quantities of hydrogen or methane if they are not separated at a FCC vapor recovery unit by compression and distillation of FCC off-gas. Typically, the ethane and lighter gases from the FCC do not undergo further separation—rather they are sent to a fuel gas system in the refinery. In any event, fixed-bed processes will incur an energy penalty when the ethylene feed purity is below about 83 mol %.
The energy penalty includes additional energy which must be used in the ethylbenzene plant when ethylene sources used are very dilute. For example, in the ethylbenzene plant described above, additional energy may be needed to recover aromatics from vent gases. Such additional processing may involve refrigerated vent condensers and/or an absorption/stripping system with reboilers and condensers.
In addition to being used for ethylbenzene production, ethylene is commonly reacted with butenes to produce propylene. Polymer-grade ethylene is most suitable for this process and will result in efficient propylene production. A number of processes for producing propylene from butenes are known, such as catalytic cracking and metathesis in fixed bed systems. The fixed bed metathesis process generally involves reacting ethylene and butenes in a reactor to produce a propylene product and, by refrigerated distillation, fractionating any unreacted ethylene so that unreacted ethylene may be recycled to the reactor for further reaction with butenes. Simultaneously, small amounts of light gases, e.g., ethane, methane and hydrogen, may be vented to prevent build-up during the recycle of ethylene.
Ethylene feeds as dilute as about 60 mol % may be used to produce propylene. Unfortunately, an ethylene feed which is more dilute than about 95 mol %, will result in a very inefficient recycle step, which comprises heating, cooling and fractionation of the unreacted dilute ethylene, because after an initial reaction with butenes the unreacted ethylene is generally diluted by relatively high levels of ethane and/or other light gases which build up in the recycle step. The result is that it is more difficult and more costly to recover ethylene in the recycle step and, thus, it may be more efficient to simply purge significant amounts of ethylene from the process along with the light gases rather than returning the ethylene to the reactor. Consequently, when a dilute ethylene feed is used, ethylene left over after the initial reaction with the butenes would be essentially unusable and not worth the energy cost of the recycle step. Although the unreacted ethylene could be returned to the ethylene plant for recovery, this would be very costly.
Nevertheless, it would be advantageous to reduce the need for high purity ethylene in propylene plants and ethylbenzene plants so as to reduce the energy spent on ethylene final fractionation in the ethylene plant. Further, there is a need for utilizing dilute sources of ethylene to produce ethylbenzene and/or propylene without significant waste or energy penalties.